Electronically controlled automatic transmissions (stepped automatic machines and CVT) for motor vehicles need engine-torque equivalent signals for shift pressure control and shift pressure regulation of the clutches and/or for contact pressure control or contact pressure regulation of the variator. The more accurate said engine-torque equivalent signals are, the more exactly can be calculated and then hydraulically adjusted the clutch pressures or variator contact pressures required for positive transmission. In this manner, on one hand, the hydraulic system pressure can be adjusted as low as possible and thus the pumping power of the transmission can be reduced in favor of efficiency and, on the other hand, shift operations of the clutches can be more comfortably adjusted. Another advantage results, specially for a CVT, by noticeable improvements in efficiency of the variator by controlling the slip limit. A CVT is a transmission having one variator for continuous adjustment of the drive ratio. A known design is a continuously variable variator with a first beveled pulley pair on an input shaft and a second beveled pulley pair on an output shaft. Each beveled pulley pair consists of a first pulley stationary in its axial direction and one second pulley movable in axial direction, respectively called also primary pulley and secondary pulley. Between the beveled pulley pairs runs a continuously variable organ such as a sliding link belt. The primary pulley and secondary pulley are adjusted by a pressure medium; to this end an electronic control unit controls via electronic actuators and hydraulic valves the pressure level of the adjustment chambers of primary pulley and secondary pulley according to input torque and the driver's desired ratio adjustment.
The precision required from the engine-torque equivalent signal applies here both to the absolute value and to the dynamic characteristic of the engine torque, e.g. during a load change.
Engine-torque dependent signals, which are known already, are based on the thermodynamic foundations of the combustion characteristic of the prime mover. The torque generated on the crankshaft as function of the induced average pressure is either measured and deposited in the engine control as a performance graph for the engine rotational speed, the air mass, the injection time or the throttle valve position, or is calculated as thermodynamic pattern. On the basis of this performance graph and the actual operation parameters, the engine control delivers an actual engine torque signal to other control units such as the transmission control, however, the actual torque thus produced depends on numerous disrupting influences and/or tolerances thus is relatively inaccurate.
EP A 584 457 has disclosed a device for regulated engagement and disengagement of the master clutch in the transmission of a motor vehicle comprised of one engine and one eletrohydraulically actuatable automatic transmission in which to detect the connection between the driving device and the body, the disconnection zone of the driving device is measured compared to the body and fed to a transmission control device which adjusts the control pressure in the master clutch of the transmission in specific driving situations in a manner such that the master clutch starts to slip. The output torque of the transmission from information of the supporting force in the supporting bearings of the driving device can be used to control the master clutch instead of the disconnection zone of the driving device. The transmission is designed as an eletrohydraulically controllable automatic transmission actuatable via an electrohydraulic control device which receives it s control command from the transmission control device and form a sensor for measuring the force in the supporting bearings for the driving device. With this known device a master clutch can be engaged and disengaged, e.g. to reduce the static vibration, exclusively when the vehicle is parked in the selected gear and when the driving wheels are braked.